Until now it was standard in foundry practice, in particular in the gray iron practice, to insert cores manually into one of the form boxes of the casting mold, comprised of at least two form boxes, which are designed for the production of thin-walled castings, e.g. motor blocks. In particular for the production of motor blocks made of gray iron, increasingly higher requirements must be met with respect to complexity of the motor block, weight savings, accuracy of measurements and surface quality. With respect to core production, this makes it necessary to produce cores with the highest possible filigree and with a very complex geometry, thereby reducing the subsequent processing of the motor blocks to a minimum. It has proven useful in this case to design the respective motor block such that bores, openings or the like, which so far had to be added in a subsequent processing step, are added from the start during the casting process. Owing to this, neighboring cores must have corresponding projections, which "hold open" these openings in the casting mold. However, it has turned out in this connection that burrs develop at the produced openings as a result of unavoidable wear and tear and as a result of deviations in the accuracy that are also unavoidable, especially in this area of contact between neighboring cores. These burrs must subsequently be reworked following the removal of the casting from the mold. It is the object of the invention to create a process that simplifies the insertion of the cores into such a casting mold and avoids a costly subsequent reworking.